The invention relates to a piston machine with changeable displacement according to the preamble of claim 1.
In multi-piston machines the capacity or piston displacement is varied in known manner in that the number of active pistons and cams is influenced (Poclain hydraulics). For changing over the multi-piston engines must be provided with a three-passage distributor which communicates with the pistons. One passage is always connected to the high pressure and a second passage to the low pressure. The third passage is switched via a switching valve. If the third passage is connected to the high pressure the machine has the full displacement. If however the third passage is connected to the low pressure, the displacement is reduced to half, the torque being correspondingly diminished. If however the same delivery volume is furnished by the fluid source the speed of the engine is doubled.
The pistons subjected at half displacement to the low pressure cause high mechanical frictional looses and furnish a starting torque which frequently no longer suffices to start up under load. As is known, the low pressure in closed hydraulic systems can lie in the order of magnitude of 15 to 25 bar and more. The efficiency losses of such machines at half displacement are then relatively high.
The invention is therefore based on the problem of further developing a piston machine with changeable displacement in such a manner that the efficiency is improved and the flow losses reduced.